1. Field of the Present Invention
The present invention relates to post-filter processing of an input sound signal in order to adjust gain values. Such gain-control processing is applicable to hearing prostheses, telecommunications, and the like.
2. Related Art
Automatic Gain Control (AGC) systems are commonly used in audio processing systems (e.g., audio headsets, hearing prostheses, etc.) to cope with a large range in sound levels. In some systems, the audio signal is split into multiple frequency bands by a filter bank of discrete components or a transform (e.g., a Fast Fourier Transform). The gain of each band can then be controlled separately. This is referred to as a multi-band type of AGC.
A variety of hearing prostheses exist to assist people who suffer hearing loss. Some are entirely external devices, e.g., conventional hearing aids. Some hearing prostheses are implantable, and more particularly are examples of an active implantable medical device (AIMD). An AIMD is a medical device having one or more implantable components, the latter being defined as relying for its functioning upon a source of power other than the human body or gravity, such as an electrical energy source. Amongst hearing prostheses, an example of an AIMD is a cochlear implant system, which is used to treat sensorineural hearing loss by providing electrical energy directly to the recipient's auditory nerves via an electrode assembly implanted in the cochlea. Electrical stimulation signals are delivered directly to the auditory nerve via the electrode assembly, thereby inducing a hearing sensation (or percept) in the implant recipient.
When fitting a cochlear implant system to a recipient, the appropriate stimulation levels for each electrode must be determined. The lowest stimulation current that is perceptible is known as the threshold level or T level. The highest stimulation current that is comfortable is known as the maximum comfortable level or C level. The T and C levels vary between recipients, and also vary between electrodes in a single recipient.
The ratio between the C and T levels on an electrode is known as the electrical dynamic range, and is typically about 10 dB. This is much smaller than the dynamic range of sound levels in the environment, and hence the processing for a cochlear implant system generally incorporates some form of compression.